GRA's have many applications and, for example, may be employed to drive aircraft flight control surfaces; however, GRA's would be even more extensively used if concerns about potential loss of flight surface control due to gear mesh jams could be alleviated, even though occurrences of such jams are very rare.
In, for example, U.S. Pat. No. 4,856,379, a non-jamming rotary mechanical actuator is proposed which includes at least one moving ring gear having an internal gear surface, with a fixed ring gear on each side of the at least one moving ring gear. Each fixed ring gear is provided with an internal gear surface defining a ring gear bore. A shaft extends through the ring gear, with the shaft including a first cam member rotatable with and secured thereon, and with an axis of the first cam member being offset from an axis of the shaft by a first amount of offset. The ring gears are maintained in a desired relationship by a retaining means which are rotatable with the shaft. Bearing means are mounted within the retaining means for rotatably supporting the shaft, and a generally cylindrical second cam member is provided having a bore extending therethrough. An axis of the bore is offset from an axis of the second cam member by a second amount of offset, with the first cam member being disposed within the bore of the second cam member and having a first amount of offset thereof approximately equal to the second amount of offset. A shear means transmits a torque from the first cam member to the second cam member below a desired torque level. A needle bearing is located around the second cam member, and a compound gear is secured over the needle bearing. The compound gear is provided with a number of gear tooth surfaces thereon, with the number of gear tooth surfaces being equal to a total number of fixed and moving gear rings and in driving relationship therewith below the desired torque level of the shear means.
The above described patented construction senses increased torque in the event of a jam and uses the abnormal torque to shear a drive key to free the input through shaft to continue to drive other actuators in the system, while at the same time disengaging the output stage of the jammed actuator to free the actuator from the control surface. A disadvantage of this approach resides in the fact that a shear key at the input of a high ratio actuator does not offer a very controllable shear setting, and may also be fatigue prone.
A further disadvantage of the above patented construction resides in the fact that the disconnecting of the output also imposes additional loading on adjacent actuators requiring all actuators in the system to be larger and heavier to accommodate potential disconnects of the other actuators.
U.S. Pat. No. 4,721,016 proposes a multiple stage geared rotary actuator having a plurality of balanced compound differentials providing a multiple stage output and for converting a high speed, low torque input to a low speed, high torque output for moving a member such as an aircraft flight control surface.
The last mentioned patented construction offers a means to reduce internal component deflection thus increasing the load capacity but does not provide for a failure mode protection.
In U.S. Pat. No. 4,742,730, a failsafe rotary actuator is proposed having irreversible dual load paths between input and output members in order to achieve a failsafe stiff link with a control surface for retaining the same in position in the event of a single mechanical failure.
While the last mentioned construction protects against an open failure, such construction is not applicable to a gear jamming action.
Another example of a geared rotary actuator is described in, for example, U.S. Pat. No. 4,825,723 wherein multiple ring gears are driven by a set of planet gears and coupled to multiple loads, with two of the ring gears being coupled to a common load through mechanical reaction paths of differing lengths, a shorter of which incorporates a compliant gear member decreasing a stiffness or spring rate of the shorter path to correspond to the stiffness of the spring rate of the longer path.
While the last mentioned patented construction offers an internal spring to balance torque on reaction ring gears, the proposed compound planetary gear assembly does not address failure modes.
Examples of other types of torque responsive overload arrangements are proposed in, for example, U.S. Pat. Nos. 4,365,962, 3,499,511, 3,898,817, 3,968,705, 4,282,776, 4,601,218 and 4,189,960.
Additionally, in commonly assigned United States application Ser. No. 590,929 entitled "Jam Tolerant Geared Rotary Actuator With Automatic Disconnect", filed on even date herewith, an alternative approach or solution for flight control surfaces utilizing a geared rotary actuator is proposed wherein, in the event of a jam, an output torque path of the actuator is automatically disengaged without any loss of position authority or torque capacity of the actuator.
Additionally, commonly assigned co-pending United States application Ser. No. 492,128 proposes a multiple prime mover system that sums inputs from two motors at the output of the actuator to provide torque continuity in the event of a jam, with reduced speed capability provided by the remaining operable motor.